Near-infrared (NIR) spectroscopy occupies a specific spot across the field of bioscience and related disciplines. Its characteristics and application potential differs from infrared (IR) or Raman spectroscopy. This vibrational spectroscopy technique elucidates molecular information from the examined sample by measuring absorption bands resulting from overtones and combination excitations. Recent decades brought significant progress in the instrumentation (e.g., miniaturized spectrometers) and spectral analysis methods (e.g., spectral image processing and analysis, quantum chemical calculation of NIR spectra), which made notable impact on its applicability. This review aims to present NIR spectroscopy as a matured technique, yet with great potential for further advances in several directions throughout broadly understood bio-applications. Its practical value is critically assessed and compared with competing techniques. Attention is given to link the bio-application potential of NIR spectroscopy with its fundamental characteristics and principal features of NIR spectra.

This review article focuses on the principles and applications of miniaturized near‐infrared (NIR) spectrometers. This technology and its applicability has advanced considerably over the last few years and revolutionized several fields of application. What is particularly remarkable is that the applications have a distinctly diverse nature, ranging from agriculture and the food sector, through to materials science, industry and environmental studies. Unlike a rather uniform design of a mature benchtop FTNIR spectrometer, miniaturized instruments employ diverse technological solutions, which have an impact on their operational characteristics. Continuous progress leads to new instruments appearing on the market. The current focus in analytical NIR spectroscopy is on the evaluation of the devices and associated methods, and to systematic characterization of their performance profiles.

By near-infrared (NIR) spectroscopy and anharmonic density functional theory (DFT) calculations, we investigate five kinds of saturated and unsaturated carboxylic acids belonging to the group of short-chain fatty acids: propionic acid, butyric acid, acrylic acid, crotonic acid, and vinylacetic acid. The experimental NIR spectra of these five kinds of carboxylic acids are reproduced by quantum chemical calculations in a broad spectral region of 7500-4000 cm and for a wide range of concentrations. By employing anharmonic GVPT2 calculations on DFT level, a detailed interpretation of experimental spectra is achieved, elucidating structure-spectra correlations of these molecules in the NIR region. We emphasize the spectral features due to saturated and unsaturated alkyl chains, the location of a C═C bond within the alkyl chain, and the dimerization of carboxylic acids. In particular, the existence of a terminal C═C bond leads to the appearance of highly specific NIR bands. These pronounced bands are located at wavenumbers where no overlapping with other structure-specific bands occurs, thus making them good structural markers. Most of the spectral differences between these two groups of molecules remain subtle, and would be difficult to reliably ascribe without quantum chemically calculated NIR spectra. Moreover, anharmonic DFT calculations provide insights on the manifestation of hydrogen bonding through distinctive spectral features corresponding to cyclic dimers. The resulting spectral baseline elevation is common for all five investigated carboxylic acids, and remains consistent with previous results on acetic acid.

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The ongoing miniaturization of spectrometers creates a perfect synergy with the common advantages of near-infrared (NIR) spectroscopy, which together provide particularly significant benefits in the field of food analysis. The combination of portability and direct onsite application with high throughput and a noninvasive way of analysis is a decisive advantage in the food industry, which features a diverse production and supply chain. A miniaturized NIR analytical framework is readily applicable to combat various food safety risks, where compromised quality may result from an accidental or intentional (i.e., food fraud) origin. In this review, the characteristics of miniaturized NIR sensors are discussed in comparison to benchtop laboratory spectrometers regarding their performance, applicability, and optimization of methodology. Miniaturized NIR spectrometers remarkably increase the flexibility of analysis; however, various factors affect the performance of these devices in different analytical scenarios. Currently, it is a focused research direction to perform systematic evaluation studies of the accuracy and reliability of various miniaturized spectrometers that are based on different technologies; e.g., Fourier transform (FT)-NIR, micro-optoelectro-mechanical system (MOEMS)-based Hadamard mask, or linear variable filter (LVF) coupled with an array detector, among others. Progressing technology has been accompanied by innovative data-analysis methods integrated into the package of a micro-NIR analytical framework to improve its accuracy, reliability, and applicability. Advanced calibration methods (e.g., artificial neural networks (ANN) and nonlinear regression) directly improve the performance of miniaturized instruments in challenging analyses, and balance the accuracy of these instruments toward laboratory spectrometers. The quantum-mechanical simulation of NIR spectra reveals the wavenumber regions where the best-correlated spectral information resides and unveils the interactions of the target analyte with the surrounding matrix, ultimately enhancing the information gathered from the NIR spectra. A data-fusion framework offers a combination of spectral information from sensors that operate in different wavelength regions and enables parallelization of spectral pretreatments. This set of methods enables the intelligent design of future NIR analyses using miniaturized instruments, which is critically important for samples with a complex matrix typical of food raw material and shelf products.

During NIR 2019 conference, Gold Coast, Australia, a presentation upon a critical review of instrumentation and applications of handheld spectrometers was delivered during the plenary session held on Thursday morning, 19 September. Following the conference presentation, a vivid discussion flared up among the audience that equally involved academic scholars, industry representatives, as well as professionals who carry out every day in-the-field applications. Various aspects were raised connected with the emerged new generation of near-infrared instrumentation, with many individuals expressing their point-of-view on the merits and pitfalls of the miniaturized spectrometers. This vigorous dispute and exchange of impressions indicated that the community remains concerned about the applicability of such devices. That concern reflects the still relatively shallowly explored miniaturization versus performance factor, which can only be dismissed by focused feasibility studies with comparative analyses carried out on scientific-grade benchtop spectrometers. It is the aim of the present manuscript to summarize the discussed scientific content and to share the developed point-of-view with addition of our remarks.

Conformational isomerism of aliphatic alcohols with respect to the internal rotation of C-O(H) group and its impact on near-infrared (NIR) spectra has been known in the literature. However, no attempt has ever been made to investigate systematically whether and how the conformational flexibility of the aliphatic chain determines the observed NIR data of aliphatic alcohols. In the present study NIR spectra of four kinds of butyl alcohols, 1-butanol, 2-butanol, isobutanol, and tert-butyl alcohol, were investigated in diluted (0.1 M) CCl solutions. The experimental NIR spectra of butyl alcohols were accurately reproduced and explained in a fully anharmonic DFT study by means of generalized second-order vibrational perturbation theory (GVPT2). Entire conformational populations were taken into account in each case. On the basis of the theoretical study, influences of conformational flexibility with respect to internal rotations not only about the C-O bond, but also about the C-C bonds have been well evidenced in the experimental spectra. The conformational isomerism affects significantly the shape of NIR spectra. The temperature-dependent NIR spectra of butyl alcohols show changes in the band shape and a blue-shift of the overtone band due to the stretching mode of free OH group, and its intensity decreases with increasing temperature. These effects can be closely monitored by two-dimensional correlation spectroscopy (2D-COS). In this work, the experimental 2D-COS patterns have been successfully reproduced, based on DFT calculated NIR spectra of conformational isomers of the studied molecules and their Boltzmann coefficients over the corresponding temperature range. Thus, the experimentally observed effects are fully reflected in the DFT study, which leads to the conclusion that the main factor in the temperature-dependent spectral changes of 2ν band of aliphatic alcohols in the diluted phase, where no self-association occurs, is played by the changes in the relative population of their conformational isomers.

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